GHC wiki contains an introductory article about STG written by Max Bolingbroke:
The STG machine is an essential part of GHC, the world’s leading
Haskell compiler. It defines how the Haskell evaluation model should
be efficiently implemented on standard hardware. Despite this key
role, it is generally poorly understood amongst GHC users. This
document aims to provide an overview of the STG machine in its modern,
eval/apply-based, pointer-tagged incarnation by a series of simple
examples showing how Haskell source code is compiled.